1. Field of the Invention
The present invention relates generally to a sprocket equipped with cushion body, and more particularly to such a sprocket which is equipped with a cushion body capable of reducing striking noise at the engagement points between a sprocket and a chain when a wrapping angle of the chain relative to the sprocket exceeds 180xc2x0.
2. Description of the Related Art
In an automobile engine, for example, in order to drive a camshaft, a chain is trained around a crank sprocket and a cam sprocket. FIG. 4 shows the relationship between such camshaft-driving chain C and the crank sprocket 50. In the sprocket 50 of this type, a cushion body 52 may be sometimes used as a noise suppression measure. The cushion body 52 is mounted on the sprocket 50 in such a manner that it comes in contact with the chain C at both an engagement start point and an engagement finish point of the chain C relative to the sprocket 50.
The chain C, as it is in the position indicated by the solid lines shown in FIG. 4, has a wrapping angle smaller than 180xc2x0. In this position, the cushion body 52 comes into contact with the chain C at the two engagement points (i.e., the engagement start point and the engagement finish point) of the chain C relative to the sprocket 50. The cushion body 52 has a ring-like shape with uniform thickness and is floatingly mounted on a boss 54 of the sprocket 50. The cushion body 52 endeavors to reduce striking noise at the engagement points between the chain C and the sprocket 50 while it is constrained in position relative to the sprocket 50 at three points, one being a contact point between the cushion body 52 and the boss 54 and the other two being contact points between the cushion body 52 and the chain C.
However, since the cushion body 52 is not prevented from moving in a direction perpendicular to the axis the sprocket 50, the position of the cushion body 52 relative to the engagement points between the chain C and the sprocket 50 is not always constant. This may lead to an insufficient noise-reducing effect achieved by the cushion body 52.
In the case where a crank sprocket of a smaller diameter is used, a greater chain wrapping angle may become necessary. In addition, when the chain C is arranged to further drive an oil pump, an alternator or the like auxiliary device disposed near the crank sprocket 50, the resulting arrangement may necessary have a greater chain wrapping angle. In these cases, the wrapping angle of the chain C exceeds 180xc2x0, as indicated by the phantom lines shown in FIG. 4. While the phantom-lined chain C is running, a downward force is applied from the chain C to the cushion body 52, tending to separate the cushion body 52 from the boss 54. The cushion body 52 thus displaced or offset downward from predetermined position is no longer possible to redice striking noise at the engagement points between the chain C and the sprocket 50. Rather, the downwardly offset cushion member per se forms an additional noise source. This problem becomes critical as the diameter of the sprocket becomes small because a sprocket of a smaller diameter is accompanied by a greater chordal action (also called xe2x80x9cchordal effectxe2x80x9d) of the chain.
In the general engine mounting structure, the crank sprocket 50 is disposed at a lower position in the vertical direction. In this case, the cushion body 52 is mounted such that a space is defined between the cushion body 52 and the chain C at a position below the sprocket 50. Since the cushion body 52 has an own weight which acts in the same direction as the downward force applied from the chain C, the cushion body 52 is likely to move in an offset direction. Especially if the cushion body 52 is displaced or offset from the redetermined position due to its own weight and a downward component force of the chain tension after the engine is stopped, restarting of the engine will immediately produce great striking noise caused due to direct contact between the chain C and the sprocket 50 and floating movement of the cushion body 50.
In addition, since the cushion body 52 is not fixed in position relative to the sprocket 50, the cushion body 52 needs to be displaced in a predetermined position when the chain C is set or trained around the sprocket 50. Thus, the efficiency of a chain-and-sprocket assembling work is low.
It is accordingly an object of the present invention to provide a sprocket equipped with a cushion body which is set in a predetermined position and, hence, is capable of surely reducing noise at engagement points between a chain and the sprocket when a wrapping angle of the chain relative to the sprocket exceeds 180xc2x0.
To achieve the object, there is provided according to the present invention a sprocket equipped with a cushion body mounted on the cylindrical outer surface of a coaxial boss of the sprocket for reducing striking noise at engagement points between the sprocket and a chain wound around the sprocket, wherein the cushion body has a solid structure including a cylindrical inner surface slidable on the cylindrical outer surface of the boss and a cylindrical outer surface eccentric to the cylindrical outer surface of the boss, the cylindrical outer surface of the cushion body being in contact with the chain at the engagement points between the sprocket and the chain.
In the state where the chain is wound around the sprocket equipped with the cushion body, the chain is in contact with the cushion body at the engagement points between the sprocket and the chain. When the wrapping angle of the chain relative to the sprocket exceeds 180xc2x0, the cushion body is subjected to a component force of the chain tension acting on each engagement point in a direction toward a top of the sprocket. In this instance, however, since the cushion body has a solid structure, movement of the cushion member is prohibited in all directions except the circumferential direction even if any force is applied to the cushion body. Thus, the cushion body can retain its prescribed posture or position.
When the sprocket starts rotation, the chain comes into meshing engagement with the sprocket at the engagement points. At the same time, the cushion body is in contact with the chain at the engagement points between the chain and the sprocket to thereby reduce striking noise between the chain and the sprocket. The cushion body is in sliding contact with the boss so that it can retain its prescribed position in which the cushion body is always in contact with the chain at the engagement points.
Even if the wrapping angle of the chain relative to the sprocket exceeds 180xc2x0, the cushion body, owing to its solid structure as opposed to the hollow structure of the conventional cushion body, can retain its prescribed position against forces (including a component force of the chain tension and the action of the gravity) tending to move the cushion body in a direction perpendicular to the axis of the sprocket. Thus, the cushion body achieves its prescribed function to reduce striking noise at the engagement points between the chain and the sprocket.
The cushion body may be comprised of a single cushion member of a disk-like configuration having an eccentric through-hole defined by the cylindrical inner surface of the cushion body and an outer peripheral surface forming the cylindrical outer surface of the cushion body. The single cushion member is simple in construction and can be produced at a relatively low cost.
As an alternative, the cushion body may be comprised of an annular bearing slidably mounted on the cylindrical outer surface of the boss and having a cylindrical outer surface, and an eccentric cushion member attached to the cylindrical outer surface of the bearing and having a cylindrical outer surface forming the cylindrical outer surface of the cushion body.
The cushion body is slidable relative to the boss and hence is not fully prevented from moving in the circumferential direction of the boss. Thus, due to a frictional force acting between the chain and the cushion body, the cushion body tends to move in the circumferential direction of the aprocket. This may cause slight oscillation or wobbling of the cushion member about the axis of the sprocket which makes it impossible for the cushion body to achieve engagement with the chain at the correct engagement points, resulting in an insufficient noise-reducing performance of the cushion body.
To make sure that the cushion body can engage the chain at the correct engagement points even when subjected to a force tending to rotate the cushion member about the axis of the sprocket, it is preferable that the cushion body further includes an annular second cushion member slidably mounted on the cylindrical outer surface of the cushion member and having an outer peripheral surface forming the cylindrical outer surface of the cushion body. With this arrangement, since the cushion body itself does not oscillate about the axis of the sprocket and the second cushion member rotates on and along the periphery of the cushion body, the second cushion member is always in contact with the chain at the engagement points between the chain and the sprocket. Thus, striking noise at the engagement points between the chain and the sprocket can be reduced sufficiently.
It is also preferable that a ring-like flange is press-fitted on the cylindrical outer surface of the boss so as to prevent the cushion body from moving in the axial direction of the sprocket while allowing sliding movement of the cushion body relative to the boss. The cushion body thus positioned and link plates of the chain are tangent internally at the engagement points between the chain and the sprocket.
The sprocket and the cushion body are preferably connected together in an assembled condition by a connector pin fitted in a pair of aligned holes formed in the sprocket and the cushion body, respectively. This arrangement is particularly advantageous when the chain is to be wound around the sprocket with two such cushion bodies mounted on both sides of teeth on the periphery of the sprocket. The cushion body are rotatable relative to the boss of the sprocket. However, since the two cushion bodies and the sprocket are connected together in an assembled c condition by means of the connector pin, relative rotation between the cushion bodies and the sprocket does not take place. Thus, the chain can readily be wound around the sprocket t while the sprocket and the cushion bodies are kept in the assembled condition.
In the case where the sprocket is used as a crank sprocket in an automobile engine, owing to the eccentricity of the cushion bodies, respective thick portions of the cushion bodies tend to turn downward until they assume their lowermost position. This movement may hinder accurate positioning of the cushion bodies relative to the chain when the chain is set to wrap around the sprocket. In addition, one of the cushion bodies is disposed between the sprocket and an engine block and hence is uneasy to manipulate. However, by connecting together the cushion bodies and the sprocket in an assembled condition by means of the connector pin, the chain can readily be wound around the sprocket while the cushion bodies are kept in a correct position relative to the chain. This may pose a remarkable increase in the assembling efficiency of a sprocket chain drive.